Do animals exhibit placebo effects?

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Question

When animal drugs testing is reported, they often compare the efficacy of an active drug to that of a placebo. I have always wondered whether there are any examples of animals actually exhibiting placebo effects (perhaps more intelligent mammals) or whether these reports should more accurately refer to a 'control group'. Do animals ever consider/appreciate the effects of medical interventions? Thanks

Answer

Could your dog be fooled by fake pills? To find out Laura Brooks asked Eleanor Drinkwater, animal behaviour researcher at York University.

Eleanor - When talking about placebo effect we tend to think about cases where just the belief in a treatment can make you feel a certain way, even when the treatment has no actual effect. It therefore seems counterintuitive to think of animals showing a placebo effect when they do not necessarily understand the aim of the treatment and can't, therefore, show belief in the outcome.

However, current research suggests that the placebo effect is not just caused by one mechanism but rather by a range of different mechanisms. One mechanism in particular for which there is strong evidence in animals is conditioning. In conditioning, an active substance that elicits a response can be repeatedly paired with an inactive substance, like a smell or a taste. Eventually the inactive substance alone can elicit the response of the active substance.

One great example, is actually, the first description of this from Pavlov who is very famous for his experiments in dogs. Now, in this particular experiment he would place a dog in a chamber and inject it with morphine. After a few days of injections, he found that just placing the chamber would result in the dog physiologically reacting as if it had been injected.

Laura - It seems that with conditioning dogs can experience placebo effects and, if that isn't surprising enough, Eleanor told me that at least in rats it can go as far as altering immune function.

Eleanor - One incredible example of this was shown by Ader and Cohen back in 1975. In their remarkable study in rats they showed that by pairing a distinctive taste with injections of immunosuppressant they could condition an association between the taste and the change in the immune system. This meant that the conditioned rats would show a dampened immune response if they were given the taste alone, even without the active immunosuppressant.

So overall, while it might be difficult to determine if animals can show a placebo effect through certain mechanisms like expectancy or belief, there is strong evidence for animals exhibiting a placebo effect through other mechanisms like conditioning.